1. Field of the Invention
In an optical sending apparatus for generating wavelength-division multiplexing optical signals by a polarization crossing method, this invention relates to an optical sending apparatus that compensates in advance for a chromatic dispersion occurring in the wavelength-division multiplexing optical signals in an optical transmission line. The present invention relates also to a channel extension method for extending afresh channels for the wavelength-division multiplexing optical signal.
To build up a future multimedia network, an ultra-long distance, high capacity optical transmission system has been required at present. A wavelength-division multiplexing (hereinafter called xe2x80x9cWDMxe2x80x9d) system has been examined as a system for achieving a large capacity due to the advantages of this system in that the system can utilize effectively a broadband large capacity of an optical fiber.
Particularly recently, the capacity of WDM optical signals must be increased so as to cope with the increase of traffic. It has therefore become necessary to narrow the spaces between the optical signals (channels) of the WDM optical signals to achieve a high density of the WDM optical signals.
2. Description of the Related Art
The WDM optical signals sent from a sending station deteriorate before they are received by a receiving station due to non-linear optical phenomena such as four-wave mixing, cross-phase modulation, and so forth, that develop in an optical transmission line such as an optical fiber. Four-wave mixing and mutual phase modulation exert greater influences on degradation of the optical signals when the spaces are smaller between the optical signals interacting with each other and when their polarization conditions are closer to each other.
Therefore, the WDM optical signals are subjected to a process that non-polarizes time-wise the polarization condition or a polarization crossing method that causes the optical signal in the polarization condition of the adjacent channels to cross orthogonal to each other, before they are outputted to the optical transmission line.
In this non-polarization process, polarization scrambler that changes time-wise retardation of mutually crossing polarization components non-polarizes the WDM optical signal. The polarization crossing method generates the optical signal corresponding to each channel under the same polarization condition. Each optical signal is guided by a polarization-maintaining fiber (hereinafter called xe2x80x9cPMFxe2x80x9d) to an optical multiplexer (hereinafter called xe2x80x9cMUXxe2x80x9d). When this PMF is connected to MUX, each PMF is connected to MUX in such a fashion that a stress-imparted portion of each PMF crosses orthogonal to that of other between PMF for transferring the adjacent channels. As a result, the WDM optical signal by the polarization crossing method is generated.
When the spaces between the optical signals of the WDM optical signals in the non-polarization method are too small, however, phase modulation by the polarization scrambler broadens the spectrum of each optical signal with the result of the occurrence of cross-talk between the adjacent optical signals. A band-pass optical filter in the receiving station, the center wavelength of the bandwidth of which is brought into conformity with the wavelength received by an optical receiver, removes the broadened spectrum component. In consequence, the reception sensitivity deteriorates. For these reasons, it is not easy to apply the non-polarization process using the polarization scrambler to the high-density WDM optical signals having the narrowed spaces.
On the other hand, chromatic dispersion develops in the WDM optical signals sent from the sending station through the optical transmission line such as the optical fiber before they are received by the receiving station. Several methods are known for compensating this chromatic dispersion, such as a method that provides a dispersion compensator having a chromatic dispersion value having an opposite sign to that of the chromatic dispersion value occurring in the optical transmission line to the optical sending station or to the optical receiving station, and a method that divides the chromatic dispersion occurring in the optical transmission line at a suitable ratio, allocates them to two dispersion compensators, provides these dispersion compensators to the optical sending station connected to the input terminal of the optical transmission line and to the optical receiving station connected to the output terminal of the optical transmission line.
Japanese Unexamined Patent Application Publication No. Hei 8-095095 discloses a dispersion compensator that compensates the chromatic dispersion without causing the influences of the polarization mode dispersion.
In the polarization crossing method, since it is necessary to maintain the polarization condition even after compensation of the dispersion until multiplexing is done by MUX. Therefore, the dispersion compensation fiber must maintain the polarization condition. Since the dispersion compensation fiber is generally elongated, however, a polarization extinction ratio remarkably deteriorates. Further, a dispersion compensation fiber capable of maintaining the polarization is difficult to produce.
The elongated dispersion compensation fiber invites a remarkable loss in the propagating optical signal, and remarkably deteriorates the optical signal-to-noise ratio of the optical signal.
It is an object of the present invention to provide an optical sending apparatus capable of generating WDM optical signals with a polarization crossing method dispersion of which is compensated in advance.
It is another object of the present invention to provide a channel extension method for extending channels afresh in WDM optical signals generated by the polarization crossing method.
These objects can be accomplished by an optical sending apparatus including a plurality of optical signal generating sections, a dispersion compensating section and a wavelength multiplexing section, wherein the dispersion compensating section gives a predetermined chromatic dispersion to at least one of a plurality of polarized light generated by the optical signal generation sections, while maintaining a predetermined state of polarized light. And the wavelength multiplexing section combines output light outputted from the optical signal generating sections with each other, or combines these output light and the output light passing through the dispersion compensating section in such a fashion that polarized light of adjacent wavelengths as the output cross orthogonal to each other.
The dispersion compensating section comprises, for example, an optical device for outputting light inputted to a first port to a second port, and outputting light inputted to the second port to a third port, a dispersion compensating device having a predetermined dispersion value and connected to the second port of the optical device, and a polarization converting section for having input polarized light outputted from the dispersion compensating device, rotating polarized light as the output of the dispersion compensating device and inputting it again to the dispersion compensating device.
Optical components such as an optical amplifier, a band-pass filter or an optical attenuator may be interposed between the optical device and the polarization converting section. Optical components such as an optical amplifier, an optical attenuator or a polarizer, each capable of maintaining polarization, may be interposed between the optical device and the wavelength multiplexing section.
The objects described above can be accomplished also by a channel extension method for multiplexing an optical signal having wavelengths which are on either the short or the long wavelength side of the wavelength band of the WDM optical signal, wherein the wavelengths are a predetermined space away from an outer edge of the wavelength band, with a WDM optical signal, after dispersion-compensating it. The multiplexing is done in such a fashion that the optical signal in the polarization condition crosses orthogonal to the optical signal of the WDM optical signal which is in the polarization condition and has the closest wavelength to that of the optical signal which was dispersion-compensated.
The optical sending apparatus having such a construction can generate high-density WDM optical signals generated by the polarization crossing method, the dispersion of which is compensated in advance. The optical communication system using this optical sending apparatus can transmit the signals for a long distance by transmitting the WDM optical signals generated by the polarization crossing method, the dispersion of which is compensated in advance.
Since the channel extension method extends the channels in such a fashion as to cross orthogonal to the existing WDM optical signal with predetermined spaces between each of the channels, this method can suppress polarizing-hole burning and the non-linear optical effects even after the channels are extended.
Furthermore, the dispersion compensating section can suppress degradation of the optical signal-to-noise ratio resulting from the dispersion-compensating fiber. It is particularly suitable for dispersion-compensating the WDM optical signal generated by the polarization crossing method.